Alternator design is known in the art. It is a fundamental principle of physics that when a magnet rotates in a wire loop, a current is induced. A magnet has a south pole and a north pole. Assume that the north pole is just passing a top part of the wire loop and the south pole is just passing the bottom part of the loop. When the magnet has rotated through 180 degrees, the south pole will be passing the top part of the loop while the north pole will be passing the bottom part of the loop. This causes the direction of induced current to be reversed. In this way, alternating current is induced in each turn of wire in a stator of an alternator.
In an alternator, a rotor is spun inside a stator. The stator includes multiple windings of wire. A single turn would not induce enough voltage nor carry enough current for typical applications of an alternator. Therefore, a practical alternator has a stator with many turns of wire.
The rotor defines an electromagnet that provides a magnetic field that is spun inside the windings of wire to generate current. A relatively small field current used to define the electromagnet is supplied to the rotor by two small brushes that each ride on separate and continuous slip rings. Field current passes through the brushes into the slip rings into the rotor.
There are typically three separate windings of wire in the stator arranged so that the AC (alternating current) that is generated by each winding is slightly out of phase compared to the other windings. This smoothes the electrical output of the alternator.
A rectifier circuit including diodes is used to convert the AC to DC (direct current). The diodes are arranged so that current from each of the three stator wires is only allowed to pass in one direction, and the three outputs are connected together. A voltage regulator is typically provided to the DC output to keep the output voltage relatively steady. The voltage regulator can be a mechanical or solid state device.
For externally regulated alternators, there are typically four connections on the alternator: the output terminal (often labeled BAT), the ground terminal (often labeled GRD) or ground may be “implied” though the metal mountings of the alternator, the field connection (often labeled F), and separate connections to each of the three poles on the stator (R).
Internally regulated models also have four connections, but the voltage regulator is inside the alternator and constructed of solid-state components. For internally regulated alternators, the connections are: an output terminal (typically labeled BAT), a ground terminal (typically labeled GRD) or ground may be “implied” though the metal mountings of the alternator, and two connections typically labeled 1 and 2. One of these connections is a relatively small wire that is connected to a battery and the other is connected to a charge indicator light.
Brushes that ride against the slip rings of the rotor of an alternator are components that are likely the number one failure mode of an alternator since the brushes wear out over time due to friction. Such brushes are conventionally internal, and are housed inside the housing of an alternator. For conventional alternators, in order to changes brushes, the alternator must be removed from service and substantially disassembled. The brush blocks then have to be removed from inside of a rear shell housing component after the rear shell has been removed from the rest of the alternator.
Certain alternators are known in the art that have removable, externally accessible, brush blocks. However, in these designs, the brushes extend out past the end of the main housing. In these designs, the rear bearings of the alternator are axially inside of the slip rings and the brushes.
Certification of components for aircraft use is a lengthy process. Components used in alternators for aircraft have subtle differences when compared with alternators used in automobiles in view of the different environments in which they are used and more serious consequences of failures in aircraft environments. For example, different brush materials are used for alternators used in aircraft than the material used in automotive alternators.
An aircraft alternator designed to deliver a certain level of amperage cannot simply be used on an airplane designed for a lower amperage alternator. For example, an 80 Amp alternator cannot be used on a 40 Amp airplane even though a regulator will regulate the current down to 40 Amps. The problem is that aircraft wiring is typically geared around the maximum rating of the alternator.
For example, forty years ago, when some of these planes were built, 40 Amp alternators were the biggest alternators available. Therefore, the gauge of the wiring going from the alternator was geared around that rating. If higher amperage current, such as 80 Amps, was passed through, the wiring could burn up. Provided that the regulator is working correctly, this would not happen. However, regulators sometimes fail and fields sometimes short. Safety standards for aircraft dictate that an aircraft alternator cannot be capable of putting out more than the designated current. This means that different alternator designs are used in different aircraft, causing manufacturers to manufacture multiple different types of alternators and causing vendors and repair facilities to stock multiple different types of alternators.